Research

Asteroseismology: a spyglass into the interiors of stars

Asteroseismology is the study and measurement of stellar oscillations (similar to earthquakes on Earth) using variations in the brightness of stars. While the oscillations we detect are the ones at the surface, their frequencies depend on the structure of the core, where they often couple to gravity waves deep within the star. Therefore, asteroseismology is a powerful tool for learning about the insides of stars, even though we will never be able to see them directly.

Using asteroseismology to find stellar mergers

When certain red giants with degenerate cores (\lesssim1.5\,M_\odot) merge with main sequence stars, they can produce more massive red giants which still have degenerate cores. For such massive red giants, degenerate cores cannot be produced by standard stellar evolution, and indicate that a red giant has undergone a merger or some kind of mass transfer in the past.

We demonstrate that the measurable “period spacing” of red giants can be used to identify such merger remnants, providing an exciting new probe for these violent events millions of years after they happen.

This work was conducted with Jim Fuller at Caltech.

In certain cases, mergers involving red giants can produce stars with unusual cores which encode their violent histories. Asteroseismology can bring that information to the surface.

Oscillations in magnetized stars

stay tuned 🙂

Modeling globular clusters in the Milky Way

Globular clusters are old, low-metallicity star clusters whose high core stellar densities place them at the center of many astrophysics problems of intrigue. Though simulating the hundreds of thousands to millions of stars is difficult, the advent of Hénon-style Monte Carlo codes such as Cluster Monte Carlo (CMC) have allowed for the creation of large model grids over realistic parameters. By fitting to the surface brightness and velocity dispersion of star clusters, I show that CMC Cluster Catalog (available here), a grid of models created using CMC, provides an excellent match to more than 40% of known globular clusters in the Milky Way, right out of the box.

Globular clusters are an incredibly vibrant field of research and have been reviewed countless times. An incredibly inspiring is review of star cluster dynamics given by The Gravitational Million-Body Problem by Douglas Heggie and Piet Hut.

Slides for the talk I gave for an Illinois Space Grant seminar can be found here. This work was conducted at the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) at Northwestern University under the Illinois Space Grant with Fred Rasio and Kyle Kremer.

Other collaboration work:

Solid-state defects as nanoscale sensors

Solid-state defects are nanoscopic “mistakes” in an otherwise regular crystal lattice. As quantum objects inbuilt into the structure of the material, defects have accessible energy levels which probe their local environments. I interfaced with instrumentation for identifying isolated nitrogen-vacancy (NV) centers, and conducted a literature search to determine that NV centers are a highly promising nanoscale sensor of magnetic fields in high pressure experiments. With Yonna Kim, I measured the response of silicon-vacancy (SiV) centers to uniform pressures, in a first step to demonstrate their promise as sensitive strain sensors.

The NV center is highly studied and lies at the center of a rich subfield in AMO physics, with Lilian Childress’s thesis serving as an excellent comprehensive review. The SiV center is somewhat less well-studied, but Christian Hepp’s thesis provides a great reference.

This work was conducted by Norman Yao’s group. I am gracious for the mentorship of Satcher Hsieh as well as many others.

emu
NV centers push the frontiers of high-pressure magnetometry to unprecedented sensitivities and spatial resolutions.

Dynamical structure of the Quintuplet Cluster

The Quintuplet cluster is a young star cluster which lies very close to the Milky Way’s center, a region characterized by extreme tidal forces, temperatures, and magnetic fields, as well as enigmatic young star formation. By using the Hubble Space Telescope to watch which stars move with the cluster, we identify members of the Quintuplet in a highly crowded and reddened field. By comparing the Quintuplet to its younger cousin, the much more compact Arches cluster, we provide evidence for the dissolution of star clusters over just a few million years.

The field of young massive clusters is summarized well by Simon Portegies Zwart’s classic review.

Slides for the Departmental Lunch Talk I gave on 21 March 2019 can be found here. Our proper motion study of the Quintuplet can be found here. This work was conducted with Professor Jessica Lu (UCB) and Matt Hosek (UCLA).

quintupletimage.png
Three-color image of the Quintuplet cluster, where the RGB values represent intensities in three NIR filters on HST WFC3-IR (120”×120”).